The manufacture of acetic acid by carbonylating methanol in the presence of a catalyst is of major industrial importance as acetic acid is employed in a wide variety of applications. While the reaction per se can be represented byCH3OH+CO→CH3COOHthe underlying chemistry is intricate and involves a multiplicity of interrelated reactions, by-products, and equilibria. To be practicable, a manufacturing process, therefore, has to balance those reactions, the associated by-products, and the purification of the product.
Prior to 1970, acetic acid was produced using a cobalt catalyst. A rhodium carbonyl iodide catalyst was developed in 1970 by Monsanto. The rhodium catalyst is considerably more active than the cobalt catalyst, which allows lower reaction pressure and temperature. Most importantly, the rhodium catalyst gives high selectivity to acetic acid.
One of the problems associated with the original Monsanto process is that a large amount of water (about 14% by weight of the reaction mixture) is needed to produce hydrogen in the reactor via the water-gas shift reactionCO+H2CO2+H2 
Water and hydrogen are necessary to react with precipitated Rh(III) and inactive [Rh4(CO)2] to regenerate the active Rh(I) catalyst. However, a large amount of water increases the formation of hydrogen iodide. Hydrogen iodide is a necessary intermediate in the reactions involved in the formation of acetic acid. However, increased amounts of hydrogen iodide are undesirable because the corrosive nature of hydrogen iodide causes engineering problems. Additionally, hydrogen iodide is involved in the formation of undesired by-products, in particular long chain alkyl iodides such as hexyl iodide, which are hard to separate from the acetic acid product. Further, removing a large amount of water from the acetic acid product renders the process more costly.
In the late '70s Celanese modified the carbonylation process by introducing lithium iodide to the reaction mixture. Lithium iodide increases the catalyst stability by minimizing side reactions which produce inactive Rh(III) species. Consequently, the amount of water which is necessary to stabilize the catalyst can be reduced. Additionally, lithium iodide has been found to decrease the vaporization tendency of water, see, e.g., EP 506 240. The process, thus, has advantages with regard to the separation of water and acetic acid. However, the respective process modification does not alleviate the problems associated with hydrogen iodide.
In the early '90s, Millennium Petrochemicals developed a new rhodium carbonylation catalyst system that does not use a metal iodide as catalyst stabilizer. Instead, the catalyst system employs a pentavalent Group 15 oxide such as triphenylphosphine oxide as a catalyst stabilizer. The Millennium catalyst system not only reduces the amount of water needed but also increases the carbonylation rate and acetic acid yield, see, e.g., U.S. Pat. Nos. 5,817,869 and 6,031,129.
Further attempts to ensure stabilization of the catalyst while reducing the amount of water employed in the carbonylation of methanol involve the use of ionic liquids, i.e., phosphonium or ammonium iodides, which are liquid under the conditions of the carbonylation reaction. The ionic liquids are proposed as catalyst stabilizer, i.e., EP 391 680, U.S. Pat. Nos. 5,416,237, and 7,115,774, or as solvent, i.e., U.S. Pat. Nos. 6,916,951, 7,115,774. While the ionic liquids are described to be advantageous in as much as they allow a further reduction of the water content, these processes have the disadvantage that ionic liquids are expensive and the processes employing them, thus, are uneconomical and not competitive.
In general, acetic acid is produced in a plant which can be conveniently divided into three functional areas, i.e., the reaction, the light ends recovery, and the purification. In general, the reaction area comprises a reactor or reaction zone and a flash tank or flash zone. The light ends recovery area comprises a light ends distillation column or fractioning zone and a phase separation vessel, e.g., a decanter. The light ends distillation column may also be part of the purification area, which in turn further comprises a drying column and optionally a heavy ends distillation column.
The primary purpose of the flash tank or flash zone is to separate the catalyst from the crude reaction mixture. The light ends distillation column or fractioning zone is critical to the process as it serves two purposes, namely, (i) to purify crude acetic acid, and (ii) to recover hydrogen iodide which otherwise may be lost from the process. Ideally, hydrogen iodide is recovered with the bottom stream formed in the fractioning zone. The bottom stream usually comprises acetic acid, water and hydrogen iodide. Hydrogen iodide forms a high boiling azeotrope in acetic acid solutions having greater than about 5 wt. % water. If the water concentration in the bottom stream falls below about 5 wt. %, azeotropic breakdown and hydrogen iodide volatilization occurs. Such volatilization leads to less hydrogen iodide in the bottom stream returning to the reaction section and, thus, may adversely impact reactor iodide inventory. Also, volatilized hydrogen iodide becomes part of the aqueous acetic acid stream which is withdrawn from the fractioning zone for further purification. Process equipment generally used in the manufacture of acetic acid is substantially inert to the components. However, the equipment may be corroded or otherwise adversely affected when the amount of hydrogen iodide in the purification section reaches excessively high levels. Additionally, hydrogen iodide gives rise to the formation of long chain alkyl iodide impurities such as, e.g., hexyl iodide, which are hard to remove and which complicate the purification of acetic acid. Thus, the presence of significant amounts of hydrogen iodide in the aqueous acetic acid which is recovered from the fractioning zone has consequences both in terms of corrosion of purification vessels and in terms of hydrogen iodide and alkyl iodide contamination of the final acetic acid product.
Accordingly, there continues to be a need to further improve the carbonylation of methanol to produce acetic acid. In particular, there continues to be a need to reduce or to eliminate entertainment of hydrogen iodide in the crude acetic acid which is recovered from the fractioning zone.